Diamond is an allotrope of carbon, where the carbon atoms are arranged in a face-centered cubic crystal structure called a diamond lattice. Crystalline diamond can be classified based on grain size into super nano-crystalline (grain size less than 10 nm), nano-crystalline (grain size between 10 nm and 250 nm), micro-crystalline (grain size from 250 nm to 250 microns) and single crystalline diamond (grain size greater than 250 microns). The diamond can exist in thin films (film thickness less than 20 microns), thick films (film thickness greater than 20 microns to 200 microns), or bulk materials (thickness greater than 200 microns). Diamond films and bulk materials can coexist with graphitic sp2 phases. Single crystal diamond films can have (100), (110), (111), or other crystal orientations.
Diamonds are adapted for many uses because of exceptional physical characteristics. Notable are its extreme hardness and high thermal conductivity (900 to 2,320 W·m−1·K−1), as well as a wide bandgap and high optical dispersion. The thermal conductivity of diamond is significantly higher than any other known material, which makes diamond thin film substrates an ideal choice for thermal challenges posed by applications including (1) miniaturization of electronic devices, (2) high brightness light emitting diodes (LEDs), (3) laser diodes and (4) high power (e.g., 500 W/mm2)/high frequency devices, (5) high frequency devices, and (6) acoustic devices. Applications well-suited for diamond layers include MEMS (micro electro mechanical structures), NEMS (nano electro mechanical structures) and diamond-based power electronic devices. Diamond can also be used for conditioning the pads in chemical mechanical polishing (CMP) processing, or as a cutting tool material.
To harness the unique properties of diamond, in applications such as for electronics, it is desirable to have ultra-smooth diamond surfaces since ultra-smooth surfaces decrease friction, increase thermal conductivity, and improve integration compatibility. The applications will generally depend on a combination of properties. For example, application of diamond as a substrate for electronic packaging takes advantage of its very high thermal conductivity for efficient heat dissipation, very high electrical resistivity for excellent electrical insulation and low permeability for environmental protection of the devices.
However, despite its desirability, ultra-smooth diamond surfaces have remained an unmet need. As known in the art, as-deposited low pressure vapor phase diamond films are very rough, with a typical average surface roughness of at least 4 nm up to several hundred microns, depending on the thickness of the film and the grain size of the film.
Traditional CMP methods have not generally been suitable for polishing of diamond films. This is primarily because of the extreme hardness and chemical inertness of diamond, which results in standard chemistries and particle-based CMP achieving very low or essentially no removal rate for diamond. Non-CMP methods have also been disclosed for planarization of diamond, including (1) laser polishing (2) ion beam polishing, (3) polishing using molten salts, and (4) diamond-diamond abrading. Such methods are neither cost-effective, nor viable for manufacturability at an industrial level. Further, such processes do not reduce the average surface roughness to a level suitable for use in semiconductor device manufacturing, such as <20 nm root mean square (rms). Moreover, the use of diamond particles without chemical additives can create sub-surface damage which can result in poor quality epitaxial growth thereon and reduced thermal conductivity.